After centuries of trying to make sense of numbers and observations, researchers may finally have discovered the time of year volcanoes are most likely to erupt. We're in it now.

On the edge of the Gulf of Alaska, surrounded by serene wilderness, North America's most active volcano regularly builds toward bursting. Pavlof has erupted 40 times in just over 200 years, heaving tons of ash and Volkswagen-size chunks of glowing debris skyward. Unlike many active volcanoes, this one isn't much of a threat to anyone. The nearest towns, King Cove and Cold Bay, are more than 30 miles away and have populations of less than 1,000 people combined. Still, Pavlof is one of the most-studied volcanoes in the world because its restless rumblings offer a good laboratory to help geologists understand how to predict when other volcanoes are likely to kill.

Steve McNutt, a boyish, bearded volcanologist at the University of Alaska, has participated in this Pavlof vigil for more than two decades. Early on in his watch, McNutt noticed that the volcano's eruptions followed a surprisingly regular pattern. Periods of activity seemed always to occur in fall or winter. Twelve of the 16 eruptions since 1973 have occurred in the autumn—four of them during the same five-day period in November. Pavlof's most recent episode, in 1996, touched off in September and peaked in December.
To be certain these observations were not mere coincidence, McNutt applied standard statistical tests to the volcano's known history. "It's absolutely not random: Pavlof doesn't erupt every fall, but when it erupts, it tends to erupt in the fall," he says. That tendency hinted at a strange, previously unknown regularity built into our planet. Maybe volcanic eruptions, like hurricanes and influenza outbreaks, are more likely to strike at certain times of the year. Perhaps there is a volcano season.

Researchers at the University of Cambridge in England had a similar epiphany recently while studying eruption patterns of another volcano 3,000 miles across the Pacific Ocean. Ben Mason, then an undergraduate in geologic sciences, was poring over data for Sakura-jima, one of the most active volcanoes in Japan, when he noticed the volcano had a strong tendency to erupt in December, January, and February. Wondering if that could be part of a widespread pattern, Mason investigated the Smithsonian Institution's mammoth global catalog, which contains more than 8,000 volcanic eruptions recorded over the last 10,000 years. He focused on volcanoes that had erupted 20 or more times since the beginning of the 18th century and identified 35 of them that accounted for more than 1,200 eruptions. Many of these highly active sites displayed an intriguing uptick in activity during winter months, Mason found. "I was struck by the clear seasonal signal. There seemed an obvious preference for Northern Hemisphere winter eruptions for many volcanoes," he says.

Mason's graduate adviser at Cambridge, volcanologist David Pyle, was intrigued but skeptical. For more than a century, scientists have been hunting for patterns to volcanic eruptions—anything that would make Earth's outbursts easier to anticipate. In the last 500 years alone, volcanoes have taken more than 200,000 lives. In 1902 Mount Pelée poured superheated gas and volcanic ash onto the city of St. Pierre on Martinique, burying the town and killing nearly 30,000 people in a matter of minutes. As more and more people choose to live closer and closer to volcanoes—such as Etna on the Italian island of Sicily and Mount Rainier outside the suburbs of Seattle—forecasting eruptions has become crucial. But volcanoes, like earthquakes, are frustratingly difficult to predict: Nobody knows for sure what triggers that moment when the slow shuffling of Earth's interior triggers an abrupt and catastrophic flare-up.

Geologists have spotted tantalizing hints of hidden seasonal order before, only to see the evidence disintegrate when examined closely. For instance, some of the most memorably destructive eruptions in recent times have occurred during the Northern Hemisphere's spring or summer. Mount St. Helens blew its top on May 18, 1980; Mount Pinatubo belched an enormous ash cloud over the Philippines on June 15, 1991; and Krakatau in Indonesia erupted on August 26, 1883, triggering tsunamis that killed more than 36,000 people. Attempts to find a general predisposition of volcanoes to come to life in the summer months failed, however.
Richard Stothers, a geologist at NASA's Goddard Institute for Space Studies in New York City, made an ambitious search for a volcano season during the late 1980s, a few years after the Smithsonian published its first version of the master eruption catalog. Like others before, he came up empty-handed. At the time, the Smithsonian database comprised 5,564 eruptions. Stothers analyzed 501 of the most explosive blasts that occurred between 1500 and 1980. He found no statistically significant pattern. Most statisticians do not trust results unless they are at the 99 percent confidence level. Even when Stothers tried lowering the bar to 95 percent, the eruptions seemed to be scattered all over the calendar.

Now Mason, a college student, was telling Pyle that Stothers and others had missed something remarkable: a volcano season that falls in the winter, not the summer. Many famous eruptions seemed to fit Mason's seasonal pattern, including the March 1982 blast from El Chichón and the December 2000 eruption of Popocatépetl, both in Mexico. Pyle started working with Mason to retrace Stothers's steps, this time using newer data. They obtained a copy of the Smithsonian Institution's newest eruption catalog and, with the help of mathematician Tim Jupp of the BP Institute at Cambridge, ran rigorous statistical analyses on the entire data set. This time, the results were exciting. Mason and Pyle found the rate of eruptions was 18 percent higher than average during the Northern Hemisphere's winter months, from December through March, a result reliable to the 99 percent confidence level.

Why didn't Stothers see this bump when he did his analysis over a decade earlier? The updated Smithsonian catalog contains thousands more eruptions than the version Stothers studied. Also, the Cambridge team looked at eruptive events of all sizes, whereas Stothers confined his search to major eruptions. "Nobody was ever able to prove it was statistically significant before," says Stothers. "Now they've done it. I'm certainly pleased by their results."

Mason and Pyle, however, worried about the results because they could not make sense of them. Some Japanese geologists had proposed that heavy winter snows create pressures that could cause a volcano to come uncorked. That mechanism might explain some wintertime eruptions. But what, the Cambridge researchers asked, could encourage volcanoes in both hemispheres to erupt between December and March, when it is frigid in Alaska and hot in Chile?

McNutt is confident a better understanding of the global pattern of eruptions will emerge from his painstaking analysis of eruption data from Pavlof. "This volcano is acting strange for a reason," he says. "We need to figure out what that reason is, because it may have something to do with the controlling factors that make other volcanoes erupt when they do." Indeed, McNutt thinks he has found the trigger: seasonal weather. In the fall, powerful low-pressure systems march across the Aleutian Islands from west to east. As the lows move in over the Gulf of Alaska, there is less air pushing down on the water, so the local sea level rises. Meanwhile, air always moves away from high pressure toward low pressure. When high-pressure systems settle in over the Pacific Ocean, strong winds rush air toward the low pressure over the Gulf of Alaska, dragging in more water. "The net effect is that water is being pushed up against the Gulf of Alaska, and there's nowhere to escape, so it piles up," McNutt says. Sea levels at this time of year end up six inches higher than average as a result. The weight of that extra water, McNutt theorizes, triggers eruptions in the fall and winter: "According to our calculations, it's squeezing the chunk of real estate beneath the volcano and pushing the magma out like a hand on a toothpaste tube."

Six inches of extra water may not seem like much, especially compared with the daily tides in the Gulf of Alaska, which can be as much as six feet. But how the additional pressure is applied might make a significant difference. The hot rock beneath Pavlof behaves something like Silly Putty. When it is pulled apart gradually, it stretches like taffy. When it is yanked apart quickly, however, it acts brittle and breaks. Likewise, the material under Pavlof acts like a solid in response to strong, quick forces created by daily tides, but it gradually shifts in response to slow force from the rising sea levels due to autumnal low-pressure systems, which persist for months. "We imagine it to be like a framework of solid rock with some kind of pore spaces or channels that are filled with molten rock," McNutt says. "On short timescales, you whack that thing and it behaves mostly like solid rock. Put a slow steady pressure on it for long periods, and the magma starts moving."
If this process really does trigger eruptions, then a similar seasonal phenomenon should influence other coastal volcanoes around the world. The effect is subtle, and it initially eluded McNutt. Then, after years of searching, he found three other volcanoes to study that tend to erupt in the fall and early winter: øOshima and Miyake-jima in Japan and Villarrica in Chile. He suspects something similar to the forces acting on Pavlof manipulates those other volcanoes as well.

While McNutt focused on specific volcanoes, the Cambridge team was searching for a mechanism that could drive volcano season around the world. Initially they suspected tides caused by the pull of the sun and the moon, but they could find no good correlation between tides and eruptions. In collaboration with another Cambridge geologist, Brian Dade, Mason and his colleagues independently began to suspect that the trigger had something to do with large-scale seasonal movements of water—Earth's hydrologic cycle.
Because the Northern Hemisphere has significantly more landmass at middle to high latitudes than does the Southern Hemisphere, a far greater amount of snow falls on land in the northern half of our planet. As a result, more of Earth's water is locked up in snow and ice between November and February than during the rest of the year. In those months, global sea levels drop by almost half an inch, leaving less weight to push down on the ocean floor. "The resulting change in stress, we thought, might be able to trigger eruptions. Most volcanoes are located near edges of continents or are islands and so would be particularly susceptible," Mason says.

At first blush, Mason's explanation seems to contradict the one championed by McNutt because it involves a decrease rather than an increase of the weight bearing down on the base of the volcano. What really matters, however, is that both teams find that volcanoes respond to slow, subtle pressure changes. Taken together, the results suggest that any long-term variation in pressure—lighter or heavier—may be a trigger for an eruption.

As their studies came together, the Cambridge researchers were anxious about how their colleagues might respond to their news. Then geophysicist Geoffrey Blewitt of the University of Nevada and his colleagues published research in the journal Science showing that Earth's shape changes on a yearly cycle. As snow, ice, and rainwater collect in the Northern Hemisphere during winter, the North Pole is pushed down a tenth of an inch. At the same time, the equator bulges out by half that much, and the entire Southern Hemisphere is pulled northward. In all, about one trillion tons of Earth's mass moves northward. "The strains are very small, much smaller than other types of strain on a local scale such as the effects of tides, but the seasonal strain is sustained over several months," Blewitt says. His paper appears to bolster Mason and Pyle's hunch that Earth's hydrologic cycle can deform the planet and perhaps nudge an unsteady volcano toward an eruption.

Michael Rampino of New York University, who has studied many large volcanic eruptions, finds Mason's reasoning intriguing but not convincing. "If there are volcanoes just sitting around waiting to pop, a small change in stress might be enough to do it. The key thing is whether the small stress changes that result from this are enough to trigger volcanism. That's the sticking point, because nobody knows exactly what it takes to trigger volcanism," Rampino says. On the other hand, he sees some signs in his own research that changing sea levels could be the culprit. He has found that some of the largest eruptions during the last 85,000 years have followed times of major climate upheaval and attendant sea-level fluctuations. Krakatau's legendary eruption in 1883 followed a decade of global cooling, for instance, as did the humongous 1912 eruption of Mount Katmai in Alaska.

If Mason and Pyle are correct that a small drop in the weight of the oceans can predispose some volcanoes to erupt in winter, bigger changes in sea level might make some volcanoes blow their tops on a seasonal cycle. This could be the common thread that ties Pavlof's eruptions to the less pronounced volcano season seen in the rest of the world. "It helps to explain why Pavlof is so extreme," says McNutt. "It's got a really big sea-level signal right next to it."

"We believe that this is an important finding with a view to long-term eruption prediction," Mason says. Volcano season is a broad, statistical effect that probably will not by itself lead to specific forecasts. But the discovery of a seasonal pattern demonstrates that volcanoes can be influenced by far more subtle influences than scientists previously believed. "In the past, people have dismissed small signals. The idea that really small stresses could modulate volcanoes is not intuitive. But if you can show that a number of volcanoes behave this way, it forces you to accept that the small stresses we haven't been paying attention to may be influencing the volcanoes," McNutt says. That realization may force researchers to rethink their models of how volcanic eruptions begin. It could also help explain the cryptic behavior of prolific volcanoes, such as Kilauea in Hawaii, that have defied attempts to uncover a trigger.

A few active or high-risk volcanoes—including Pavlof, Mount Pinatubo, and Popocatépetl—are already covered with seismic or GPS sensors, but geologists monitoring these networks have mostly focused on big, short-lived signals. Other volcanoes receive even less detailed attention. "We've been taking a good hard look at part of the picture and ignoring other parts," McNutt says. The small, easily overlooked changes may be the ones that lead scientists to a deeper understanding of the bottled fire inside these mountains.